The present invention relates to motor control circuits and is directed more particularly to phase-control circuitry for controlling variables such as the voltage, current, or speed of d-c motors.
One advantageous construction for a closed-loop d-c motor control system involves the utilization of a phase-controlled rectifier network such as a thyristor bridge connected between an a-c source and the armature or one of the field windings of the motor. In such circuits, the motor is controlled by controlling the firing times of the various thyristors. When, for example, firing signals are applied to the thyristors at times which are relatively early in the a-c input voltage cycle, a relatively large average value of voltage will be applied to the motor, causing the motor to operate at a relatively high speed. When, on the other hand, firing signals are applied to the thyristors at times which are relatively late in the a-c input voltage cycle, a relatively smaller average voltage will be applied to the motor, causing the motor to operate at a relativey lower speed. Thus, the motor speed is dependent upon the angle by which the thyristor firing signals lag a selected point on the input voltage waveform or, in other words, upon the control angle.
In single phase motor control systems of the above type, a typical configuration includes a full-wave thyristor bridge connected between a single-phase a-c input and the motor armature. In such systems, the control activity may include the alternate application of firing signals to the pairs of thyristors on opposite arms of the bridge at control angles of from 0.degree. to 180.degree. of lag behind the a-c input voltage. In practice the effect of the counter EMF of the motor causes maximum drive power to be available when the thyristor firing signals lag behind the zero crossings of the a-c input voltage by 33.degree..
In three-phase motor control systems of the full-wave type, a typical configuration includes a three-phase thyristor bridge connected between a three-phase a-c source and the motor armature. In such systems, the control activity includes the application of firing pulses to various pairs of thyristors, at controllable times, in a sequence including six pairs of thyristor firings for each full cycle of the a-c input voltage. For reasons similar to those stated in connection with the single phase system, maximum drive power in three-phase full-wave systems becomes available when thyristor firings lag behind the zero crossings of the a-c input voltage bu 60.degree..
Other common controlled rectifier configurations include the three-phase half-wave configuration and the six-phase full-wave configuration. Still another common control rectifier configuration includes pairs of parallel-connected, oppositely-poled thyristors in each a-c input lead, this configuration usually being used in half-wave type drive systems which are to have the ability to reverse the direction of motor rotation. It will be understood that the present invention can be used to control any and all of the above controlled rectifier configurations.
Prior to the present invention, it was often the practice to control the firing times of the thyristors by utilizing an analog control scheme in which a traingular wave was compared against a d-c reference voltage which had a magnitude dependent upon the difference between the actual and desired values of a motor variable. In such systems, the comparison of the triangular wave with the d-c reference voltage resulted in a pulse-width modulated signal the transitions of which were utilized to control the firing times of a controlled rectifier network. Control systems of this type are described in U.S. Pat. Nos. 2,867,763 and 3,883,786. While analog control systems of this type operate satisfactorily in a number of motor control applications, they are subject to the problems of inaccuracy of control, thermal drift, and sensitivity to component tolerances. These problems may, in turn, result in the unequal sharing of load current between thyristors and cause the overheating and premature failure of the most heavily loaded thyristors.
More recently, the utilization of digital control circuitry has alleviated certain of the problems associated with analog phase control circuitry. Accompanying these improvements, however, have been new problems such as increased sensitivity to line and environmental noise, the lack of mechanism for continuously updating the feedback data and increased circuit complexity. Another problem was that a designer had to choose between having a separate set of digital controls for each thyristor firing event or having a single set of digital controls and accepting a limit in the range of angles over which the motor could be controlled. In a three-phase full-wave controlled rectifier configuration, for example, a deisgner could, on the one hand, accept a 60.degree. limit in the range of motor control, in which case he could utilize a single digital control circuit for all of the thyristors. On the other hand, the designer could provide desired 180.degree. range of motor control by providing separate digital control circuits for each of the six combinations of thyristor pairs which were to be fired.
In accordance with the present invention, there is provided digital control circuitry wherein the circuitry is highly stable and insensitive to environmental noise, where the circuitry provides continuously updated feedback data and wherein the circuitry provides a full 180.degree. range of motor control, from a single digital control circuit, without regard to the number of thyrsitors.